Method of manufacturing high pressure sodium arc discharge lamp

ABSTRACT

In the manufacture of a high pressure sodium arc discharge lamp, a glass-like frit ring is used to seal a niobium tube to an alumina washer. The washer is then sealed to the end of an alumina arc tube by means of the previously melted frit ring.

United States Patent Barakitis 1 Feb. 18, 1975 METHOD OF MANUFACTURING HIGH 3.5311853 10/1970 Klomp 29/4729 PRESSURE SODIUM ARC DISCHARGE 3,693,007 9/1972 Kcrekes 313/220 X 1726.582 4/1973 T01 ct a1 316/17 LAMP [75] Inventor: Nikolaos Barakitis, Haverhill, Mass.

. Primary ExaminerRoy Lake [73] Asslgnee' ggf g g lz Assistant Examiner.lames W. Davie Attarney, Agent, or Firm-James Theodosopoulos [22] Filed: Dec. 26, 1973 [21] Appl. No.: 427,693

. ABSTRACT [52] US. Cl. 29/25.l5, 29/4729 [51] Int. Cl. H01j 9/18 In the manufacture of a high pressure sodium arc dis- Of Search 25.15, Charge lamp a glassJike frit ring is used to ea] a niol-9, -9; 3 7, bium tube to an alumina washer. The washer is then 4/5061, 50-6215 -6 sealed to the end of an alumina arc tube by means of the previously melted frit ring. [56] References Cited UNITED STATES PATENTS 5 Claims, 3 Drawing Figures 3,088,201 5/1963 Louden et a1 29/4719 PATENTEDFEB 1 81975 FIG. I

FIG.2

FIG.3

1 METHOD OF MANUFACTURING HIGH PRESSURE SODIUM ARC DISCHARGE LAMP BACKGROUND OF THE INVENTION 1. Field Of The lnvention This invention relates to are discharge lamps and in particular to high pressure sodium (HPS) vapor lamps.

2. Description Of The Prior Art Within the past few years, high pressure sodium vapor lamps have become commercially useful, especially for outdoor lighting application because of their high efficiency, generally in excess of 100 lumens per watt. The sodium operating vapor pressure in such lamps is of the order of about 50 to 100 Torr.

Such lamps are called high pressure in order to distinguish them from low pressure sodium vapor lamps in which the sodium operating vapor pressure is in the order ofa few microns. Low pressure sodium lamps have been in use for about 30 years but, although efficient, they produce an unattractive monochromatic yellow light. The color of light from HPS lamps is considerably improved over that from low pressure sodium lamps.

HPS lamps generally comprise an alumina ceramic arc tube containing a fill including sodium, mercury and an inert gas. Disposed at the ends of the arc tubes are metal electrodes and the ends of the arc tube are sealed with ceramic to metal seals.

A common method of manufacture of the arc tube involves the use of an alumina washer. The washer is disposed on the end of the arc tube with a glass seal ring therebetween. A niobium tube, containing an electrode on its inner end, is then'inserted through a hole in the alumina washer with part of the niobium tube protrud-' ing externally. A second glass sealing ring is disposed around the niobium tube and rests on the alumina washer. The assembly is then heated to the melting point of the glass sealing rings to form the ceramic to metal seal. The first ring seals the alumina washer to the arc tube. The second ring seals the niobium tube to the alumina washer.

A problem resulting from this method of manufacturs is the deposition of an undesirable dark material at the ends of the arc tube. This deposit is undesirable since it reduces the light transmission of the arc tube. The deposit occurs as a result of a reaction between the glass sealing material and the niobium tube.

It is an object of this invention to eliminate the undesirable dark deposit during the manufacture of HPS lamps.

SUMMARY OF THE INVENTION In this invention only one frit ring is used to seal the end of an alumina arc tube instead of two rings as in the prior art. A niobium tube is inserted through a clearance hole in an alumina washer and a sealing glass-frit ring is disposed on the washer around the niobium tube. The assembly is then heated to a temperature sufficient to melt the ring and cause it to flow in the capillary space between the niobium tube and the washer. The tube and washer assembly are then placed on the end of an alumina arc tube and are then subjected to a second temperature that is higher than the first melting temperature. This higher temperature permits easier glass flow, thereby permitting the previously melted glass to flow into the capillary space between the alumina washer and the end of the arc tube.

BRIEF DESCRIPTION OF THE DRAWlNG HO. 1 is a sectional view of a niobium tube and alumina washer in position to be sealed by a glass frit ring and,

HO. 2 shows the same view after the frit ring has been melted.

FIG. 3 shows the tube and washer sealed to the end of an alumina arc tube.

DESCRlPTlON OF THE PREFERRED EMBODIMENT As shown in FIG. 1, an electrode for an HPS arc dis-. charge lamp comprises a tungsten coil I mounted on a refractory metal rod 2. Rod 2 is welded or brazed to the end of a niobium tube 3 which positions and supports the electrode within arc tube 8. The internal end of niobium tube 3 is vacuum tight sealed.

Niobium tube 3 extends through a hole in alumina washer 4 and is axially aligned therein by araisedperipheral seating ring 5 on niobium tube 3. For this purpose, the outer diameter of seating ring 5 is slightly larger than the diameter of the hole in alumina washer 4, for example, 173 mils versus mils.

Next, a frit ring 6 made of sealing glass is disposed around niobium tube 3 resting on alumina washer 4. Frit ring 6 has a composition of the type commonly used in the sealing of alumina arc tubes for HPS lamps and consists mainly of alumina and alkaline earth oxides, primarily, calcia.

The assembly, comprising niobium tube 3, alumina washer 4 and frit ring 6, is then placed in a vacuum furnace with tube 3 in a vertical position, the electrode end thereof downward. The furnace is then evacuated to ajsubmicron vacuum and sufficient heat is applied to the assembly to cause frit ring 6 to melt and flow; this temperature is about l350C. The glass, indicated as 7 in the drawing, flows completely around niobium tube 3 and into the capillary space between tube 3 and alumina washer 4 as shown in FIG. 2. Said capillary space is only a few mils thick. During this melting step, vertical downward pressure may be applied to the upper end of niobium tube 3 to maintain perpendicularity and uniform seating on alumina washer 4.

It is during this first melting step that any reaction between the sealing glass and niobium tube can occur, which results in the dark reaction product that has discolored prior art arc tubes. However, the arc tube is not present during this first melting step and, therefore, is not discolored.

After removal from the furnace, the alumina washerniobium tube assembly is then placed on the end of an alumina arc tube 8. Arc tube 8 is a substantially cylindrical tube throughout its length and has a cylindrical alumina insert 9, having an axial hole therethrough, interiorly disposed at each end. Each insert 9 has been previously sintered into arc tube 8 to form a vacuum tight joint therebetween. The purposeof insert 9 is to reduce the inside diameter of arc tube 8 at its end in order to provide a more reliable seal between the niobium tube and the arc tube.

After the alumina washer-niobium tube assembly is placed on the end of arc tube 8, they are placed in a suitable furnace and heated, in an inert atmosphere, to melt glass 7. Glass 7 is heated to a higher temperature than during the first melting, for example, l550C, and flows into the capillary space between niobium tube 3 3, andinsert 9. Glass 7 also flows into and fills the capillary space between washer 4 and the end of the arc tube, as shown in FIG. 3. As during the first melting, vertical downward pressure may also be applied at the upper end of niobium tube 3 during the second melting step. In order to ensure that frit ring 6 contains adequate material to fill all the capillary spaces by means of two melting steps, the second at a higher temperature than the first, the capillary spaces should be less than about mils thickness. That is to say, the CD. of

niobium tube 3 should be about 2 to mils less than the ID. of alumina washer 4 and insert 9. Also, the surface of washer 4 should be suitably plane and parallel to the end surface of the arc tube so that the space therebetween is less than about 5 mils.

Frit ring 6 must be of adequate size so that after said capillary spaces are completely filled during the two melting steps, sufficient glass material remains on the upper surface of washer 4 to form glass fillet 10, shown in FIG. 3, around the periphery of niobium tube 3. In one example of a 400 watt HPS lamp, where are tube 8 was 4.3 inches long by 0.350 inches diameter, niobium tube 3 was 0.158 inches in diameter, washer 4 was 0.116 inches thick and the ID. of washer 4 and insert 9 was 160 to 165 mils, frit ring 6 had a weight of 80 mg which was sufficient to fill the capillary spaces and provide fillet 10 in accordance with this invention.

I claim:

1. In the manufacture of a high pressure sodium arc discharge lamp, the steps comprising: inserting an electrode-bearing niobium tube through a hole in an alumina washer; disposing a glass-frit ring around the niobium tube; heating the frit ring to a first temperature sufficient to melt the glass and to thereby seal the niobium tube to the alumina washer; placing the tubewasher assembly on the end of an alumina arc tube which contains an interiorly disposed alumina insert sintered thereto, so that a capillary space exists between said insert and the niobium tube of said tubewasher assembly; and heating the assembly to a second temperature, higher than the first, sufficient to cause the previously melted glass to flow into said capillary space and to seal the assembly to the arc tube.

2. The process of claim 1 wherein said first temperature is about l350C.

3. The process of claim 2 wherein said second temperature is about 1550C.

4. The process of claim 1 wherein the glass seal between the niobium tube and the alumina washer is less than about 5 mils thick.

5. The process of claim 1 wherein the glass seal between the alumina washer and the are tube is less than about 5 mils thick. 

1. IN THE MANUFACTURE OF A HIGH PRESSURE SODIUM ARE DISCHARGE LAMP, THE STEPS COMPRISING: INSERTING AN ELECTRODEBEARING NIOBIUM TUBE THROUGH A HOLE IN AN ALUMINA WASHER; DISPOSING A GLASS-FRIT RING AROUND THE NIOBIUM TUBE; HEATING THE FRIT RING TO A FIRST TEMPERATURE SUFFICIENT TO MELT THE GLASS AND TO THEREBY SEAL THE NIOBIUM TUBE TO THE ALUMINA WASHER; PLACING THE TUBE-WASHER ASSEMBLY ON THE END OF AN ALUMINA ARC TUBE WHICH CONTAINS AN INTERIORLY DISPOSED ALUMINA INSERT SINTERED THERETO, SO THAT A CAPILLARY SPACE EXISTS BETWEEN SAID INSERT AND THE NIOBIUM TUBE OF SAID TUBE-WASHER ASSEMBLY; AND HEATING THE ASSEMBLY TO A SECOND TEMPERATURE, HIGHER THAN THE FIRST, SUFFICIENT TO CAUSE THE PREVIOUSLY MELTED GLASS TO FLOW INTO SAID CAPILLARY SPACE AND TO SEAL THE ASSEMBLY TO THE ARC TUBE.
 2. The process of claim 1 wherein said first temperature is about 1350*C.
 3. The process of claim 2 wherein said second temperature is about 1550*C.
 4. The process of claim 1 wherein the glass seal between the niobium tube and the alumina washer is less than about 5 mils thick.
 5. The process of claim 1 wherein the glass seal between the alumina washer and the arc tube is less than about 5 mils thick. 